DFM Your Device? Simulation Is the Answer

Design for Manufacturing (DFM) has taken off like a rocket in the electronics industry in recent years. Performing process and device simulations on CMOS transistors in a DFM flow have proven to be important in shortening the design cycle and reducing the production cost by improving yield. These are the key elements to why DFM mans the gate to the success of semiconductor foundries and companies.

What about optoelectronics? How far are we from achieiving DFM of optoelectronics? To answer this question, we need to examine the history of the development of optoelectronics innovation and simulation.

DFM Optical Challenges

Figure 1: Flowchart connecting various equations needed in the simulation of a semi-conductor laser/LED, adopted from Synopsys’ Sentaurus Device — Optoelectronics. The equations encased in the red dotted box are used for silicon transistor simulation and are only a subset of the equations needed for a laser/LED simulation.
Optoelectronics encompasses the devices that process light by means of electrical flow. At one end are the optical sensors that convert absorbed light into electrical signals. These devices are mostly based on silicon and they can be adapted easily into the DFM flow of electronics. At the other end are the active optical devices that convert electrical signals into light, such as lasers and light emitting diodes (LEDs), mainly constituted from III-V direct gap compound semiconductors. This article will focus mainly on active optoelectronics devices. Active optoelectronics devices are more difficult to simulate than silicon electronic transistors. Consider this: To simulate the working behavior of a transistor, one only needs to solve the Poisson equation, carrier continuity equations and thermal equation with relevant boundary conditions, and quantum-mechanical equations for very small transistors. To simulate a laser or LED, one needs to solve not only the same equations as for the transistor, but also equations that model electron and hole scattering into quantum wells (QWs), the probability of light production (optical gain calculations), the optical mode pattern of the laser/LED structure, and a coupling equation that balances the photon production (light treated as particles) with the electron-hole recombination (See Figure 1).

Figure 2: Parallel coordinate plot from Synopsys’ Sentaurus TFM — PCM studio option showing correlation between design variation of metal contact radius (Rm) and oxide aperture radius (Rox), and desired output characteristics of threshold current (Ith), slope efficiency (slope), single mode power (P_SM), differential resistance (R), and maximum change in temperature (ΔT).
Besides the computational burden of solving the optoelectromic problem set, the growth technology of III-V compound semiconductors was not very mature, (i.e. producing high-quality and low-defect material was a problem). Material parameters were not well known, and if material parameters could not be calibrated, it didn’t make sense to perform a complete 2D or 3D simulation. The optoelectronics industry relied mainly on experimental optimization — a costly and time-consuming process.

Calibrate OE Parameters

In the late 1990s, growth technology of III-V compound semiconductors began to reach a mature state. On top of that, computers were becoming more powerful. New models in quantum well scattering and transport were also developed. All these factors assimilate into a suitable stage for full 2D and 3D simulation of semi-conductor lasers/LEDs. The problem of a laser/ LED simulation is not only about solving a set of complicated equations simultaneously, but also about fulfilling the large parameter space associated with each device. Calibration of this parameter space is key to using the simulator as a predictive tool and hence DFM of optoelectronics.


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